1. Field of the Invention
This invention relates to a thermally developable light-sensitive material, more particularly, to a thermally developable light-sensitive material which shows decreased thermal fog (undesirable fog caused in unexposed portions upon heating) as compared to an identical element lacking rosin or a diterpene acid by incorporating therein at least one rosin or diterpene acid.
Another aspect of the invention relates to a thermally developable light-sensitive material which shows decreased thermal fog and an improvement in "green" shelf stability (the capability of retaining, even after storage, the photographic characteristics of the light-sensitive material immediately after production, especially sensitivity), particularly at high humidity, by incorporating therein rosin and/or a diterpene acid, and optionally sulfinic acid and/or a salt thereof.
2. Description of the Prior Art
The most widely used photographic process is a process in which silver halides are employed since such a photographic process provides an element with superior photographic properties, such as sensitivity and gradation, as compared to elements resulting from other photographic processes such as an electro-photographic process or a diazo process. There are, however, disadvantages in the silver halide photographic process. For example, the silver halide light-sensitive element must be developed in an aqueous developing solution after exposure and the thus developed light-sensitive element must be further subjected to several aqueous solution processings such as stopping, fixing and stabilization, in order to prevent the thus formed image from discoloring or fading and to prevent the undeveloped white areas in the image (background) from darkening. These solution processings are time-consuming and troublesome, and the chemicals employed in these processings are hazardous to handle, stain the worker's body and clothes and the processing room, and further can give rise to environmental pollution if the solutions are discharged without treatment into a stream or the like.
Therefore, a light-sensitive element which utilizes high speed silver halides which can be dry processed without using processing solutions and which is, moreover, capable of forming a stable image with minimal background discoloration under normal room illumination has been desired.
To achieve such, various efforts have been made. For example, German Pat. Nos. 1,123,203 and 1,174,157 disclose that heat development of a sliver halide light-sensitive element is possible by incorporating therein a 3-pyrazolidone type developing agent. German Pat. No. 1,175,075 disclosed that the heat-developability of such an element can be promoted by incorporating therein a material capable of forming water, and German Pat. No. 1,003,578 discloses the additional incorporation therein of a fixing agent for the silver halide. According to the above-described techniques, however, the silver halide remaining in the element after the dry processing is never completely stabilized against light, that is, the former three patents do not describe a dry fixing step and in the light-sensitive element described in the last patent it can be easily appreciated that the co-presence of a developing agent (a reductant) and a fixing agent during storage gives rise to an undesirable reaction, which makes the element in practical for use as has been experimentally confirmed.
At present, the most successful light-sensitive element capable of forming a photographic image by dry processing is a heat developable light-sensitive element employing a composition comprising a silver salt of an organic acid, a small amount of a light-sensitive silver halide and a reducing agent, e.g., as described in U.S. Pat. Nos. 3,152,904 and 3,457,075. In this light-sensitive system, the silver halide which remains in the element after development is not stabilized against light and is allowed to be discolored by light. Nevertheless, the element provides results as if it were stabilized, because the silver halide is only used in a slight amount and most of the silver source is a white or light-colored organic silver salt which is stable and hardly blackened by light. Thus, even if a small amount of silver halide is discolored by light, the slight discoloring does not give rise to any visual difficulties since the element, on the whole, still appears white or only slightly colored. The light-sensitive element is stable at normal temperature, but when it is image-wise exposed and heated to a temperature, usually above about 80.degree. C., preferably above 100.degree. C., the organic silver salt oxidizing agent and the reducing agent which are present in the light-sensitive element undergo an oxidation-reduction reaction due to the catalytic action of the exposed silver halide present in the vicinity thereof to form silver. By this reaction, the exposed areas of the light-sensitive layer are rapidly blackened so that contrast is formed between the exposed areas and the unexposed areas (background), that is, an image is formed.
This invention is concerned with an improvement in the afore-said thermally developable light-sensitive materials, particularly, it has as its object reducing thermal fog using a stabilizing agent.
Various methods have hitherto been known for preventing thermal fog of thermally developable light-sensitive materials. One method is to use a mercury compound as described in Japanese Patent Publication No. 11,113/72. However, mercury compounds are poisonous and they are undesirable in practice since the use of such involves some danger. Another method for preventing thermal fog is to use an N-halogeno compound, such as N-halogenosuccinimide and N-halogenoacetamide as described in Japanese Patent Applications (OPI) Nos. 10,724/74, 97,613/74 and 90,118/74. In addition, as one other method for preventing thermal fog, it is known to use an acid stabilizing agent, for example, higher aliphatic acids such as lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid, salicylic acid, p-hydroxybenzoic acid, tetrabromobenzoic acid, tetrachlorobenzoic acid, p-acetamidobenzoic acid, alkyl-substituted benzoic acids such as p-t-butylbenzoic acid, phthalic acid, isophthalic acid, trimellitic acid, pyromellitic acid, diphenic acid, 5',5'-methylene bissalicylic acid, etc., as described in U.S. Pat. 3,645,739 and Japanese Pat. Application (OPI) No. 89,720/73.
The above N-halogeno compounds and the acid stabilizing agents have the effect of preventing thermal fog, but their effects are unsatisfactory for practical purposes.
Moreover, these compounds, in some cases, produce undesirable effects such as a lowering of sensitivity, a deterioration of light stability and a lowering of image density, which also inhibit the practical use of such compounds.
The inventors, after extensive investigation on the above problems of the prior art achieved this invention.